This invention relates to the generation of keying signals for use with digital video signals. Keying signals or key signals are used to define regions of an image to which a certain operation is to be applied. Typically, a keying signal is used to make a newsreader who is in fact located in a studio appear in front of a picture of a remote location. The background picture may be derived from a slide or solid state store for example. A key signal may also be used to define parts of a picture in which certain colors are to be altered.
The keying signal is usually generated from a video signal itself. For example, the newsreader in a studio, mentioned above, will be seated in front of a blue backdrop. The output of a foreground camera viewing the newsreader is analysed to determine which areas of the picture are blue, and thus to generate a key signal which is used to switch in the background picture in those regions. The key signal may be termed a "matte" or "stencil", and may be single valued (ON or OFF), but is preferably multivalued to allow a fading operation particularly at the boundary of objects for example.
All this has been known in the analogue video processing field for many years.
Key signal generators in the analogue field generally use large amounts of special purpose analogue circuitry such as comparators. In the digital field they use a digital equivalent of this technique.
The present invention is concerned with the adaptation of these techniques for use with digital signals, particularly though not exclusively to digital video signals encoded in accordance with CCIR Recommendations 610 and 656. In accordance with these recommendations, out of every four signal samples, two are luma (luminance) samples, and two are chroma (chrominance) samples. The two chroma samples correspond respectively with the R-Y and B-Y color component signals of the PAL system and are associated in time with one only of the two luma samples.